L. Denti

Structural characterization of biomedical Co-Cr-Mo components produced by direct metal laser sintering.

Direct metal laser sintering (DMLS) is a technique to manufacture complex functional mechanical parts from a computer-aided design (CAD) model. Usually, the mechanical components produced by this procedure show higher residual porosity and poorer mechanical properties than those obtained by conventional manufacturing techniques. In this work, a Co-Cr-Mo alloy produced by DMLS with a composition suitable for biomedical applications was submitted to hardness measurements and structural characterization. The alloy showed a hardness value remarkably higher than those commonly obtained for the same cast or wrought alloys. In order to clarify the origin of this unexpected result, the sample microstructure was investigated by X-ray diffraction (XRD), electron microscopy (SEM and TEM) and energy dispersive microanalysis (EDX). For the first time, a homogeneous microstructure comprised of an intricate network of thin ε (hcp)-lamellae distributed inside a γ (fcc) phase was observed. The ε-lamellae grown on the {111}γ planes limit the dislocation slip inside the γ (fcc) phase, causing the measured hardness increase. The results suggest possible innovative applications of the DMLS technique to the production of mechanical parts in the medical and dental fields.

Effects of thermal treatments on microstructure and mechanical properties of a Co-Cr-Mo-W biomedical alloy produced by laser sintering.

Direct Metal Laser Sintering (DMLS) technology based on a layer by layer production process was used to produce a Co-Cr-Mo-W alloy specifically developed for biomedical applications. The alloy mechanical response and microstructure were investigated in the as-sintered state and after post-production thermal treatments. Roughness and hardness measurements, and tensile and flexural tests were performed to study the mechanical response of the alloy while X-ray diffraction (XRD), electron microscopy (SEM, TEM, STEM) techniques and microanalysis (EDX) were used to investigate the microstructure in different conditions. Results showed an intricate network of ε-Co (hcp) lamellae in the γ-Co (fcc) matrix responsible of the high UTS and hardness values in the as-sintered state. Thermal treatments increase volume fraction of the ε-Co (hcp) martensite but slightly modify the average size of the lamellar structure. Nevertheless, thermal treatments are capable of producing a sensible increase in UTS and hardness and a strong reduction in ductility. These latter effects were mainly attributed to the massive precipitation of an hcp Co3(Mo,W)2Si phase and the contemporary formation of Si-rich inclusions.

A combined additive layer manufacturing / indirect replication method to prototype 3D vascular-like structures of soft tissue and endocrine organs: A combined additive layer manufacturing (ALM)/ indirect replication method to prototype 3D vascular-like structures of soft tissue and endocrine organs is presented in this paper

We describe an innovative methodology combining Additive Layer Manufacturing (ALM) and indirect replication to reconstruct reticular-like, three-dimensional (3D) structures mimicking the vascular network of soft tissue and endocrine organs. Using a fractal-like algorithm capable of modelling the intraparenchymal vascular distribution of these viscera, single intraglandular branches of the human thyroid arteries were prototyped with synthetic resin, based on the algorithmic standard to layer (STL) output and ALM techniques. Satisfactory dimensional accuracy was obtained for these models, which were used as masters to evaluate protocols for their indirect replication, through both single and double procedures. Additional studies were conducted using casts of the human kidney arteries, obtained by injection / corrosion of the isolated organ. Satisfactory 3D reproduction of the external morphology of the kidney vessels was achieved. We conclude that our approach has the potential to develop up to the reconstruction with biomaterials of an entire, intraparenchymal vascular tree of soft tissue and endocrine organs.

Dimensional Tolerances and Assembly Accuracy of Dental Implants and Machined Versus Cast-On Abutments

BACKGROUND The clinical application of prosthetic components obtained by different manufacturing processes lacks technological foundation: the dimensional tolerance of individual parts and their assembly accuracy are not known. The rotational misfit (RM) of the hexagonal connection is critical in single-tooth implant restorations, but no standard control procedures are available for its evaluation. PURPOSE The research aimed at proposing a new protocol for the dimensional assessment of implant-abutment connections, based on noncontact measurement and statistical data processing. The procedure was applied to machined- and cast-on abutments, as well of the matching implants. MATERIALS AND METHODS Three groups of five abutments each were studied: machined titanium abutments, pre-machined calcinable abutments before casting procedures and the same specimens after casting. A group of five corresponding implants was considered as well. Twice the apothem was measured on each hexagon through an optical measuring microscope. The data were processed to obtain the international tolerance (IT) grade. The RM was then calculated using the apothems of the external and the internal hexagon. RESULTS All the components were classified between IT8 and IT9, and the maximum RM was around 3-4° for all the assemblies, inferior to the critical limits for the screw joint stability. CONCLUSION An original measuring protocol was developed, independent of parts assembly and based on ITs. An objective dimensional characterization of prosthetic components and assemblies has been achieved, which is the basis for their reliability in clinical applications.

Multi-disciplinary approach in engineering education: learning with additive manufacturing and reverse engineering

Purpose – The purpose of this paper is to report an interdisciplinary, cooperative-learning project in a second-year course within the “Enzo Ferrari” Master of Science Degree in Mechanical Engineering. The work aims to raise awareness of the educational impact of additive manufacturing and reverse engineering. Design/methodology/approach – Students are asked to develop, concurrently, the design and the manufacturing solution for an eye-tracker head mount. A digital head model is reverse engineered from an anatomical mannequin and used as an ergonomic mock-up. The project includes prototype testing and cost analysis. The device is produced using additive manufacturing techniques for hands-on evaluation by the students. Findings – Results of the presented case study substantiate the authors’ belief in the tremendous potential of interdisciplinary project-based learning, relying on innovative technologies to encourage collaboration, motivation and dynamism. Originality/value – The paper confirms a spreading ...

Assay of Secondary Anisotropy in Additively Manufactured Alloys for Dental Applications

Even though additive manufacturing (AM) techniques have been available since the late 1980s, their application in medicine is still striving to gain full acceptance. For the production of dental implants, the use of AM allows to save time and costs, but also to ensure closer dimensional tolerances and higher repeatability, as compared to traditional manual processes. Among the several AM solutions, Laser Powder Bed Fusion (L-PBF) is the most appropriate for the production of metal prostheses. The target of this paper was to investigate the mechanical and microstructural characteristics of Co–Cr–Mo and Ti–6Al–4V alloys processed by L-PBF, with a specific focus on secondary anisotropy that is usually disregarded in the literature. Tensile specimens were built in the EOSINT-M270 machine, along different orientations perpendicular to the growth direction. Density, hardness, and tensile properties were measured and the results combined with microstructural and fractographic examination. For both alloys, the results provided evidence of high strength and hardness, combined with outstanding elongation and full densification. Extremely fine microstructures were observed, sufficient to account for the good mechanical response. Statistical analysis of the mechanical properties allowed to attest the substantial absence of secondary anisotropy. The result was corroborated by the observations of the microstructures and of the failure modes. Overall, the two alloys proved to be high-performing, in very close agreement with the values reported in the datasheets, independently of the build orientation.

Investigation on the Effectiveness of Through-hole Replicas of Deep Small Holes

When a micro-hole of high aspect-ratio is required, in addition to machining problems, special attention should be paid to controlling the quality of the manufactured products. Dimensional and surface metrology in the field of micromachining can be as critical as machining itself, therefore several new measurement methods have been developed. However, many of these methods suffer from application limits when used in the case of a deep hole. Replication can be a useful approach, which has well-proven validity in the time-consuming case of the sectioned hole. The method of through replicas, to be pulled out of the unsectioned hole, still needs verification. In the present paper, the surface roughness of deep small electrodischarge drilled holes is measured, and the effectiveness of the use of both openand through-replicas is evaluated, versus direct measurements on the microdrilled surfaces. Through-hole replicas, by means of injection and extraction of a silicone, are proven reliable for reproducing the surface morphology of holes down to 0.8 mm diameter with an aspect ratio of 12.5. The findings show that the operative range of the considered techniques may be extended with respect to the previous cases mentioned in literature

DREAM: Driving up reliability and efficiency of additive manufacturing

The DREAM project, financed by the EU Commission (H2020, Work program: FOF-13-2016: Photonics Laser-based production) is an end-user driven action which aligns the research and development of Additive Manufacturing (AM) technologies to the specific needs of its three industrial end users, Ferrari SpA, Adler Ortho France SARL, and RB Srl. The Action brings together experts in the field of AM technologies, powder and material characterization, component engineering, laser-matter interaction, to deliver an optimized approach that will be developed and demonstrated to the requirements of the end users. The first results of the project are here reported.

On the Effect of Electrodischarge Drilling on the Fatigue Life of Inconel 718

Difficult-to cut-materials are associated with premature tool failure, most likely in the case of complex geometries and this shapes. However, Nickel-based alloys are commonly used in high-temperature and aerospace applications, where thin deep holes are often required. Then, the only viable manufacturing solution relies on non-contact processes, like electrodischarge (ED) drilling. Morphology of ED machined surfaces is significantly different than obtained by metal-cutting operation and is known to jeopardize fatigue strength, but the extent needs to be gauged and related to the process parameters. Aim of the paper is to study the effect of holes (0.8 mm diameter, aspect ratio 10) produced by ED drilling on the fatigue life of Inconel 718. Rotating bending fatigue tests are carried out on specimens drilled under two ED setups, as well as with a traditional cutting tool. Specimens free from holes are fatigued under the same conditions for comparison. Based on previous studies, extremal ED parameters are selected, giving best surface finish versus highest productivity. S-N curves show that the ED process causes a decrease of the fatigue resistance with respect to traditional drilling, whereas the effect of different ED setups is negligible. Maximum productivity can thus be pursued with no threat to fatigue performance. The fatigue limit variation is quantified by using the superposition effect principle: ED drilling causes an increase of the stress concentration factor around 25% if compared to traditional drilling. The macroscopic fatigue behavior is integrated with a study of the effects of the different drilling processes in the micro-scale, by means of a microstructural and fractographic analysis.